Method of making graphite molds



Feb. 26, 1946. J. HEUBERGER 2,395,403

METHOD OF MAKING GRAPHITE MOLDS Filed April 2, 1942 Patented Feb. 26,1946 2,895,408 7 ltIE'I'HOD OF MAKING GRAPHITE MQLDS Josef Heuberger,Stockholm, Sweden, assignor to Alrtiebolaget Hammarbylampan,

Stockholm,

Sweden, a company of Sweden Application April 2, 1942, Serial a...431,445

In Sweden February 15, 1941 2 Claims.

The present invention relates to a method 01 making molds from graphite.In the metal ceramic scopes of activity, there is a need for molds fromheat resistive material. For this purpose graphite has found a veryextensive use. Hitherto, hollow molds of the cup type" closed at thebottom generally are manufactured by turning, grinding or some otherworking involv ing the detachment of chips. Disregarding the loss ofmaterial, this manufacture is tedious and does not enableobtaining-surfaces as smooth as desirable; Moreover, the surfacesoperated upon are rendered rather porous, so that the products made inthe molds will obtain a rather irregular surface structure. Endeavourshave been made, therefore, to press such molds from graphite powder orfrom mixtures of graphite and carbon powder with binding agents, andthen to undertake sintering of the molds, that is to say, a

method analogous to the manufacture of carbon electrodes, carbon brushesor the like.

However, due to the fact that these graphite molds are to be pressed ashollow bodies having a bottom, the ordinary powder pressing techniquecannot be applied for the reason that the distribution of pressurewithin the powder mass becomes too irregular. This is so because ifgraphite powder or a powder mixture with appreciable contents ofgraphite be subjected to pressure, the graphite particles will orientthemselves (occupy a determined direction) relatively to the directionof pressure on account of their pronounced leaf-like structure, so thatthe powder will fiow, preferably in a direction influenced by thegraphite leaves or flakes. This flow of material along the glidingsurfaces referred to will then cause initiations of rupture in thepressed body, which are frequently so pronounced that the body cannot beremoved from its press mold, without cracks being produced.

According to the present invention, a method has been evolved whichsolves this problem and eliminates the difiiculties by the graphitepowder mass being pressed in a manner such that the pressure in thepowder mass will be the same all over in planes at right angles to thedirection of compression. Hereby the advantage is attained that lateralcreeping does not occur, so that the powder particles, disregarding thereduction of distance in the direction of pressure, will maintain theirproper mutual positions. According to the invention, this is attained,for instance, by making the stamp as a divided body, and by adapting thepressure on the various parts of the stamp, so that those portions ofthe powder mass which are actuated by the pressure will show the samedegree of compression at every moment of the pressing operation.

0n the accompanying sheet of drawings:

Fig. 1 is a diagrammatic representation of one embodiment of theinvention;

Fig. 2 shows a modified form of bottom piece.

In order properly to adapt the pressure on the various parts of thestamp,'the same powderas that from which the mold body is manufacturedmay be used as a pressure-regulating material. Such a method isillustrated in Figure l. A stamp consisting of an external member, andprovided with a top plate 8, is filled internally with graphite powder9. The stamp operates in a matrix In provided with a bottom plate II. Ifat the commencement of the pressing operation the height of the quantityof powder enclosed within the stamp is designated by A and the height ofthe quantity of powder between the inner stamp member 1 and the bottomplate I l is denoted by B, and the height between the outer stamp membert and the said plate It is designated by C, and the height is so adaptedthat A+B=C, then the condition for the obviation of creeplngs isfulfilled, that is to say, the graphite will be compressed at the sametime in every part of the press body, and there will be an equalpressure in planes extending at right angles to the direction ofpressure. In applying this method excellent results are obtained. Dueregard must be paid, however, to the fact that an additional press bodywill be formed from the powder quantity 9 in every pressing operation.

According to the method described above, the problem has been solved bythe use of a. divided stamp. However, the conditions referred to inconnection with the stamp apply in regard to the bottom piece too. In ananalogous manner also this bottom piece may be composed of mutuallymovable parts, the movement of which, taking place under the action ofpressure, is controlled by filling the body with pressing mass or by theuse of springs.

In the examples above referred to, the methods relate primarily to themanufacture of a graphite cup, wherein the distribution of pressure inthe pressing mass can be readily anticipated on account of the simplegeometry of the press body. On the other hand, if graphite molds of morecomplicated outlines are to be pressed, it is difficult and oftenpractically impossible to form an idea as to the distribution ofpressure in the graphite mass. Without a knowledge of the pressuredistribution it is impossible to construct the as for pressing thegraphite body so that the movable parts of the stamp or bottom piecewill obtain the proper shape; The flow at the compression is dependentnot only on the inner friction, but also on the friction against thewalls of the tool (the press chamber), and with complicated shapes thisfactor cannot be calculated in a simple andpracticablemanner.

Another difliculty in calculating the distribution of pressure in thepressing mass may reside in the composition of the mass proper. As longas pure graphite powder is operated with,-one knows rather well theinternal friction and the manner in which the individual graphiteparticles will be directed on account of their leaf-. like structure,the planespf the fiakes turning, into a position so as to lie at rightangles to the direction of pressure; the pressing mass is non-isotropicas regards pressure. In order to obtain greater strength or otherdesirable properties, such as hardness, porosity of the molded bodyafter the pressing operation and/or upon sin tering, it is frequentlynecessary, however, to

mix the graphite with other more or less plastic substances adapted tobind at the sintering or pressing operation in accordance with theeffect desired. Such additional substances are carbon, coke, soot, forexample, or sirup-like solutions, with which the powder is impregnated,without losing its pulverulent nature at pressing temperature, ifdesired after special drying. Inasmuch as these additional substancesgenerally do not have a leaf-like structure, the said admixture willcause the pressing mass to lose some of its property of beingnon-isotropic as regards pressure.

The properties of flow of the pressing mass will be substantiallyaltered, and the calculation of the distribution of pressure in the massduring compression is rendered particularly difllcult, if the substancesto be admixed are of such a nature stored into the orisinal shape afterthe pressing operation.

As many changes could bemade in the above method and construction andmany apparently 'widelydiflerent embodiments of the invention could bemade without departing from the scope thereof, asv defined by thefollowing claims, it is intended that all matter contained in the aboveY description shall be interpreted as illustrative and not in a limitingsense. -"WhatIclaimis:-

'1. a method of producing essentially cup-use articles from pulverulentmaterial such as graphite, which comprises confining the said materialin a pressure mold having a bottom piece, between v the said bottompiece and a vertically disposed pressure applicator longitudinallymovable in the mold, said applicator comprising at least two relativelylongitudinally movable pressure-applying parts, thelower ends of whichterminate in vertically spaced horizontal planes, which parts, duringthe compressing operation act in thesame direction longitudinally of thesaid parts and of said mold, said pressure applying parts also'defininga completely closed space therebetween confining a pulverulent materialhaving substantially the same compressibility as said firstnamedpulverulent material in the space between the relatively movable partsof th pressure applicator. the said materials being under the same stateof compression initially and the initial height that their plasticproperties are changed through the heating caused by the friction whenthe mass is compressed. This difficulty is obviated according to anothermethod in accordance with the invention by using plastic parts in thestamp or bottom piece of the tool. If, in performing the pressingoperation, a sufilclently plastic bottom piece lid is used, such bottompiece will, upon one or more pressing operations, take the shape shownin Figure 2, that is to say, the boties will be found particularlysuitable, but alsosintered press bodies of metal powder from softductile metals such as copper, to-which' the graphite body does notadhere, will be found useful. When lower pressures are operated with,obviously such elastic substances as rubber will adopt apressure-equalizing shape which is reof the said first-named pulverulentmaterial when the mold is full and the material is in the kmcompressedstate being substantially equal to the sum of the initial height of thepulverulent material between the said applicator parts and the initialdistance between the bottom of the lowermost of the said parts and thebottom piece of the mold, and then applying pressure to said firstnamedpulverulent material through the-said applicator parts and the materialcomprised therebetween, whereby a substantially equal pressure ismaintained throughout the material being compressed in planes at rightangles to the direction of compression.

' 2. In apparatus for producing essentially cuplilre articles fromcompressibl material, a pressure mold having a bottom piece, a pressureapplicator longitudinally movable in said pressure mold, said pressureapplicator comprising at least 7 two relatively longitudinally movablepressureapplying parts, the lower ends of which terminate in verticallyspaced horizontal planes, which parts, during'the compression operationact in the same direction longitudinally of the said parts and of thesaid mold, said pressure-applying parts defining a recess therebetween,said recess being filled with pulverulent material of substantially thesame compressibility as said firstnamed compressible material, thelongitudinal extent of said recess when the mold is full and saidcompressible material is in uncompressed state being substantially equalto the difference between the distances from the several lower ends ofsaid pressure-applying parts to said bottom PIECE.

J OSEF HEUBERGER.

